1. Field of Utilization
This invention relates generally to a recording medium for recording audio data in a new data format and, more specifically, to a two-layered recording medium.
2. Description of the Background
Compact disks (CD's) are now very popular as a recording media with high audio quality and typically have a diameter of about 12 cm on which digital audio data is recorded having a sampling frequency fs equal to 44.1 kHz and with 16-bit quantization.
With the progress in the audio industry and the requirement for a higher capacity and a higher transfer rate of various recording media, higher sound quality is required for the CD, which is referred to hereinafter as the first generation CD, and for realizing such higher sound quality the first approach to be considered is increasing the sampling frequency.
More specifically, the sampling frequency fs=44.1 kHz of the first generation CD restricts the frequency band component of the audio signal data up to 20 kHz, and the higher sampling frequency allows the CD to record audio data of frequencies higher than 20 kHz and a more natural fidelity sound is recorded and reproduced.
It is possible to construct a new CD media system having a data format with a higher sampling frequency than that of the first generation CD, however, such new CD media system involves various practical problems. If such new CD media system is realized, it is inevitably required that it be compatible with the first generation CD. For example, a playback device structured for new CD having a data format with the high sampling frequency is required also to playback the first generation CD. On the other, from the view point of the CD system as a recording medium, it is required that the new CD should be able to be played back on the conventional player used for the first generation CD.
Thus, although the high sound quality could be realized by simply increasing the sampling frequency, the new CD cannot be reproduced using a player for the first generation CD, so that such a new CD is not sufficient as a new CD system.
A player for the new CD system can be constructed to play back the first generation CD, however, to perform such playback, two series of digital playback circuits performing the functions of a decoder and a D/A converter for the first generation CD and a decoder and a D/A converter for the new CD system are necessary. Of course, a clock generator is also required for each circuit. Such added circuitry results in a complex, large sized, and expensive circuit structure of the playback device. Therefore, it is not a suitable solution to the problem of improving CD sound quality.